1. Field of the Invention
The present invention relates to method of producing a ceramic green sheet used for manufacturing a multilayer ceramic electronic part such as a monolithic ceramic capacitor, a multilayer varistor, or the like. The present invention also relates to a multilayer ceramic electronic part, particularly to a method of manufacturing a multilayer ceramic electronic part having a structure in which a plurality of internal electrodes are disposed with ceramic layers provided therebetween.
2. Description of the Related Art
Although a monolithic ceramic capacitor which is a typical example of multilayer ceramic electronic parts is widely used for various applications, the monolithic ceramic capacitors have, with progress of miniaturization of electronic parts, recently increasingly been required to have a small size and large capacity.
The monolithic ceramic capacitor has a structure, for example, shown in FIG. 1, in which a ceramic element 3 comprises a plurality of internal electrodes 2a and 2b opposed to each other with ceramic layers serving as dielectric layers provided therebetween, ends of the internal electrodes 2a and 2b being led to different side ends of the ceramic element 3, and a pair of external electrodes 4a and 4b are disposed at both sides of the ceramic element 3 to be connected to the internal electrodes 2a and 2b, respectively.
Ceramic green sheets used for manufacturing the monolithic ceramic capacitor are used for forming the above-described ceramic layers serving as the dielectric layers, and the ceramic green sheets have recently increasingly been thinned in order to increase the acquirable capacitance and decrease the product size.
Each of the green sheets is generally produced by forming ceramic slurry in a sheet shape and then drying. As the method of forming ceramic slurry in a sheet, various methods such as a doctor blade method, a reverse roll coater method and the like, are used.
For example, FIG. 2 shows an example of conventional methods of producing ceramic green sheets, in which a carrier film 52 is supplied from a carrier film supply unit (carrier film supply roller) 51, and a ceramic slurry 54 is coated on the carrier film 52 by a ceramic slurry coater (sheet former) (in this example, using the doctor blade method) 53 at a predetermined position, and conveyed together with the carrier film 52 to a drier 55 for drying the ceramic slurry 54 to form a ceramic green sheet 56. Then, the carrier film 52 holding the ceramic green sheet 56 formed on the surface thereof is wound by a sheet recovery roller 57 so that the formed ceramic green sheet 56 held by the carrier film 52 is recovered.
In the monolithic ceramic capacitor shown in FIG. 1, when the ceramic layers 2 interposed between the internal electrodes 2a and 2b have a thickness (element thickness) of 3 xcexcm or less, thin ceramic green sheets must be produced as ceramic green sheets used for the ceramic layers. However, in producing thin ceramic green sheets by the above-described conventional method, there are the problems of roughening the surfaces of the ceramic green sheets and producing pinhole defects (pores) in the ceramic green sheets.
Furthermore, ceramic dielectric materials such as barium titanate, strontium titanate, calcium titanate and the like, which have a perovskite structure, are conventionally widely used as capacitor materials to utilize their high relative dielectric constants. Also, as a capacitor which is a passive part, a small capacitor which can obtain high capacitance has recently been demanded with miniaturization of electronic parts.
A conventional monolithic ceramic capacitor using a ceramic dielectric material for a dielectric layer must be burned at a high temperature of about 1300xc2x0 C. in air, and thus a noble metal such as palladium must be used as an internal electrode material. However, such a noble metal material is very expensive and increases the cost ratio of the electrode material in a product. This is a main factor which hinders a decrease in cost.
Therefore, in order to solve the problem, the internal electrodes of the monolithic ceramic capacitor are increasingly made of a base metal material, and various dielectric materials which can be burned in a neutral or reducing atmosphere and which have reduction resistance are developed for preventing oxidation of electrodes during burning.
Under these conditions, the monolithic ceramic capacitor is required to have a smaller size and larger capacity, leading to the progress of development in techniques for increasing the dielectric constant of a ceramic dielectric material and thinning ceramic dielectric layers and internal electrode layers.
However, when each of the ceramic layers (the ceramic layers interposed between the internal electrodes) has a thickness (element thickness) of 3 xcexcm or less, unevenness in the interfaces between the ceramic dielectric layers and the internal electrode layers is increased, or defects (pores) in the ceramic dielectric layers are increased, thereby causing the problem of decreasing the lifetime.
In order to improve the smoothness of a ceramic green sheet used for forming the ceramic layer, and increase the density of the ceramic green sheet, a method has been disclosed in which the particle diameter of a ceramic powder material is decreased (Japanese Unexamined Patent Publication No. 10-223469).
However, as the particle diameter decreases, generally, the ceramic powder itself readily agglomerates to decrease dispersibility. Therefore, the method of decreasing the particle diameter has limits of improvement in the surface smoothness of the ceramic green sheet and increase in the density thereof. For a ceramic dielectric powder having the same composition, the dielectric constant decreases as the particle diameter decreases, thereby causing the problem of making it impossible to comply with a large-capacity monolithic ceramic capacitor.
One method of dispersing ceramic slurry comprises rotating the ceramic slurry at a high speed by a sand mill or visco mill to supply great shear force to the ceramic slurry. However, although dispersion proceeds by applying the shear force to the ceramic powder, this method has the problem of partially grinding the ceramic powder. Grinding the ceramic powder improves the surface smoothness of a sheet due to an increase in dispersibility, but characteristics are changed by grinding the ceramic powder to cause the problem of deviating the temperature characteristics of the resultant monolithic ceramic capacitor from the desired range of design, and decreasing the dielectric constant of the ceramic dielectric.
Furthermore, the number of the layers laminated must be increased while decreasing the size of the monolithic ceramic capacitor and increasing the capacity thereof, and thus a monolithic ceramic capacitor is developed in which about 300 thin ceramic green sheets having an element thickness of 3 xcexcm or less are laminated.
However, with not less than 300 layers laminated, the step formed by the thickness of an internal electrode is increased to cause the problem of producing delamination due to the step of the electrode, and the problem of bending electrode portions being extended for connecting the internal electrodes to external electrodes and producing a short-circuit defect.
These problems apply to not only the monolithic ceramic capacitor but also other multilayer ceramic electronic parts.
The present invention has been achieved for solving the above problems, and an object of the present invention is to provide a method of producing a ceramic green sheet capable of stably producing a ceramic green sheet having good surface smoothness and few pinhole defects even when producing a thin ceramic green sheet. Another object of the present invention is to provide a method of manufacturing a multilayer ceramic electronic part which is capable of securely and efficiently manufacturing a multilayer ceramic electronic part and preventing deterioration in its life due to unevenness in the interfaces between internal electrodes and ceramic layers, and the occurrence of a structural defect (delamination, bending of an electrode portion, or the like) in a multilayer thin film.
In order to achieve the objects of the present invention, a method of producing a ceramic green sheet comprises the forming step of coating ceramic slurry containing a ceramic powder dispersed in a dispersion medium on a carrier film to form a sheet, the drying step of drying the sheet-formed ceramic slurry on the carrier film, and the smoothing step of pressing the dry sheet obtained by drying the ceramic slurry on the carrier film by using a plate press comprising at least a pair of pressing plates under conditions of a pressing plate surface temperature of about 0 to 150xc2x0 C. and a pressing pressure of about 500 to 10,000 kgf/cm2 to smooth the surface of the sheet.
The dry sheet (a ceramic green sheet not subjected to smoothing process) obtained by forming the ceramic slurry in a sheet on the carrier film and then drying is pressed with the plates for each carrier film by using the plate press under conditions of a pressing plate surface temperature of about 0 to 150xc2x0 C. and a pressing pressure of about 500 to 10,000 kgf/cm2, and thus the surface smoothness of the ceramic green sheet can be improved independently of the particle diameter and dispersibility of ceramic. Therefore, a ceramic green sheet suitable for use for manufacturing a monolithic ceramic capacitor can securely be produced.
The plate press comprises, for example, a pair of parallel plates (pressing plates) each comprising a mirror-polished hard chromium plated layer provided on the surface thereof, a pressure control for controlling the pressing pressure of the pressing plates, and a heater for heating the pressing plates to a predetermined temperature so that the ceramic green sheet can be held between the pair of pressing plates, and pressurized from both sides under heating to the predetermined temperature to smooth the surface of the ceramic green sheet. The construction of the plate press is not limited.
In smoothing by the plate pressing method, ceramic green sheets may be pressed one by one, or a plurality of sheets may be stacked and pressed. However, in order to prevent the ceramic green sheet from adhering to the pressing plates, the processed surface of a film subjected to separating process is preferably attached to the surface of the ceramic green sheet, followed by pressing. In pressing a plurality of ceramic green sheets, in order to prevent each ceramic green sheet from adhering to the carrier film superposed thereon, the processed surface of a film subjected to separating process is preferably attached to the surface of each ceramic green sheet, or a carrier film on the back of which is subjected to separating process is preferably used.
In the present invention, the pressing plate surface temperature is in the range of about 0 to 150xc2x0 C. because with a temperature of less than about 0xc2x0 C., the sheet surface is hardened to deteriorate the effect of decreasing the surface roughness (Ra), while with a temperature of over about 150xc2x0 C., the sheet is softened due to thermoplasticity to transfer the sheet from the carrier film to the pressing plate side.
In addition, the pressing pressure is in the range of about 500 to 10,000 kgf/cm2 because with a pressing pressure of less than about 500 kgf/cm2, the sufficient surface smoothing effect cannot be obtained, while with a pressing pressure of over about 10,000 kgf/cm2, the ceramic green sheet is separated from the carrier film or broken, thereby making processing impossible.
The method of producing a ceramic green sheet of the present invention is preferably characterized by plate pressing under conditions of a pressing plate surface temperature of about 20 to 100xc2x0 C. and a pressing pressure of about 1,000 to 6,000 kgf/cm2. Plate pressing under conditions of a pressing plate surface temperature of about 20 to 100xc2x0 C. and a pressing pressure of about 1,000 to 6,000 kgf/cm2 can more securely improve the surface smoothness of the ceramic green sheet.
The reason for limiting the pressing plate surface temperature to about 100xc2x0 C. or less is that heating the ceramic green sheet at about 100 to 150xc2x0 C. causes transfer of the ceramic green sheet to the pressing plate side due to thermoplasticity in some cases, thereby wrinkling the surface and deteriorating the effect of decreasing surface roughness.
The reason for limiting the pressing pressure between the pressing plates to about 6,000 kgf/cm2 or less is that with a pressing pressure of about 6,000 to 10,000 kgf/cm2, wrinkles can occur on the surface of the ceramic green sheet, thereby deteriorating the effect of decreasing surface roughness.
In plate-pressing under the temperature condition and pressure condition in the ranges suitable for the ceramic green sheet, the effect of smoothing the surface of the ceramic green sheet can securely be obtained without separating the ceramic green sheet from the carrier film or breaking the ceramic green sheet.
In another aspect of the present invention, a method of producing a ceramic green sheet comprises the forming step of coating a ceramic slurry containing a ceramic powder dispersed in a dispersion medium on a carrier film to form a sheet, the drying step of drying the sheet-formed ceramic slurry on the carrier film, and a smoothing step of hydrostatically pressing the dry sheet obtained by drying the ceramic slurry on the carrier film using a hydrostatic press under conditions of a pressing temperature of about 0 to 150xc2x0 C. and a pressing pressure of about 500 to 10,000 kgf/cm2 to smooth the surface of the sheet.
A dry sheet (a ceramic green sheet not subjected to smoothing process) obtained by forming the ceramic slurry in a sheet on the carrier film and then drying is hydrostatically pressed for each carrier film by using the hydrostatic press under conditions of a pressing temperature of about 0 to 150xc2x0 C. and a pressing pressure of about 500 to 10,000 kgf/cm2, and thus the surface smoothness of the ceramic green sheet can be improved independently of the particle diameter and dispersibility of ceramic. Therefore, the ceramic green sheet suitable for use for manufacturing a monolithic ceramic capacitor can securely be produced.
The hydrostatic press comprises, for example, a pressure container filled with a liquid such as oil or water, a pressure cylinder for pressurizing the liquid, a control for controlling the pressure applied to the liquid, and a heat exchanger for heating the liquid to a predetermined temperature. For example, the ceramic green sheet is wound on the mirror-polished surface of a metal roll, packed with a flexible sheet under vacuum, and then immersed in the liquid of the hydrostatic press, such as oil or water, followed by hydrostatic pressing. As a result, the ceramic green sheet is pressed on the surface of the metal roll and the film on the back of the ceramic green sheet under uniform pressure to smooth the surface of the ceramic green sheet. The construction of the hydrostatic press is not limited, and a roll comprising a material other than a metal may be used in place of the metal roll.
In pressing a ceramic green sheet which is wound on the metal roll over and over by the hydrostatic pressing method, a carrier film having its back subjected to separating process is preferably used.
For the ceramic green sheet wound on the metal roll over and over, the thickness of the whole ceramic green sheet is increased, and thus the pressure applied to both ends of the wound ceramic green sheet cannot be neglected, thereby causing deformation in some cases. In this case, flanges are provided at both ends of the metal roll, and the ceramic green sheet is wound on the metal roll so that both ends of the ceramic green sheet closely contact the flanges to prevent an adverse effect on both ends of the ceramic green sheet.
The hydrostatic pressing may be performed by using a flat plate in place of the metal roll. In hydrostatically pressing a plurality of ceramic green sheets stacked, a carrier film having its back subjected to separating process is preferably used.
In the present invention, the pressing temperature is in the range of about 0 to 150xc2x0 C., and the pressing pressure is in the range of about 500 to 10,000 kgf/cm2 for the same reasons for the limits as the plate pressing method for smoothing.
In the method of producing a ceramic green sheet of the present invention, hydrostatic pressing is preferably performed under conditions of a pressing temperature of about 20 to 100xc2x0 C. and a pressing pressure of about 1,000 to 6,000 kgf/cm2. The hydrostatic pressing under the conditions of a pressing temperature of about 20 to 100xc2x0 C. and a pressing pressure of about 1,000 to 6,000 kgf/cm2 can more securely improve smoothness of the surface of the ceramic green sheet.
The reasons for limiting the pressing temperature to about 20 to 100xc2x0 C. and the pressing pressure to about 1,000 to 6,000 kgf/cm2 are the same as the pressing plate surface temperature in the range of about 0 to 100xc2x0 C. and the pressing pressure in the range of about 1,000 to 6,000 kgf/cm2.
As described above, hydrostatic pressing under temperature and pressure conditions in the ranges suitable for the ceramic green sheet can securely achieve the effect of smoothing the surface of the ceramic green sheet without causing separation of the ceramic green sheet from the carrier film or breakage of the ceramic green sheet.
In a further aspect of the present invention, a method of producing a ceramic green sheet comprises the forming step of coating ceramic slurry containing a ceramic powder dispersed in a dispersion medium on a carrier film to form a sheet, the drying step of drying the sheet-formed ceramic slurry on the carrier film, and the smoothing step of calendering a dry sheet obtained by drying the ceramic slurry on the carrier film using a calender roll comprising at least a pair of nip rolls under conditions of a nip roll surface temperature of about 0 to 150xc2x0 C. and a pressing pressure (linear pressure) of about 50 to 1,000 kgf/cm to smooth the surface of the sheet.
The dry sheet (a ceramic green sheet not subjected to smoothing process) obtained by forming the ceramic slurry in a sheet on the carrier film and then drying is calendered for each carrier film by using the calender roll under conditions of a nip roll surface temperature of about 0 to 150xc2x0 C. and a pressing pressure (linear pressure) of about 50 to 1,000 kgf/cm, and thus the surface smoothness of the ceramic green sheet can be improved independently of the particle diameter and dispersibility of ceramic. Therefore the ceramic green sheet suitable for use for a monolithic ceramic capacitor can securely be produced.
The calender roll comprises, for example, a preheating roll (which may not be provided in some cases) for preheating the ceramic green sheet, at least a pair of nip rolls, and a heat exchanger for heating the nip rolls so that the ceramic green sheet is held between the pair of nip rolls and pressed from both sides to smooth the surface of the ceramic green sheet under pressure. The construction of the calender roll is not limited, and various types of calender rolls such as a single nip roll-type calender roll, a multi-stage nip roll-type calender roll comprising plural pairs of nip rolls, and the like can also be used.
In the present invention, the reason for setting the nip roll surface temperature in the range of about 0 to 150xc2x0 C. is that with a temperature of about 0xc2x0 C. or less, the sheet surface is hardened to deteriorate the effect of decreasing surface roughness (Ra), while with a temperature of over about 150xc2x0 C., the sheet is softened by thermoplasticity to be separated from the carrier film and transferred to the pressing plate side.
The reason for setting the pressing pressure (linear pressure) in the range of about 50 to 1,000 kgf/cm is that with a linear pressure of less than about 50 kgf/cm, the sufficient surface smoothing effect cannot be obtained, while with a linear pressure of over about 1,000 kgf/cm, the ceramic green sheet is separated from the carrier film or broken to make processing impossible.
In the method of producing a ceramic green sheet of the present invention, calendering is preferably performed under conditions of a nip roll surface temperature of about 20 to 100xc2x0 C. and a pressing pressure (linear pressure) of about 100 to 600 kgf/cm. The calendering under the conditions of a nip roll surface temperature of about 20 to 100xc2x0 C. and a pressing pressure (linear pressure) of about 100 to 600 kgf/cm can more securely improve smoothness of the surface of the ceramic green sheet.
The reason for limiting the nip roll surface temperature to about 100xc2x0 C. or less is that heating the ceramic green sheet at about 100 to 150xc2x0 C. causes transfer of the ceramic green sheet to the pressing plate side due to thermoplasticity in some cases, thereby wrinkling the surface, and deteriorating the effect of decreasing surface roughness.
The reason for limiting the pressing pressure (linear pressure) to about 600 kgf/cm or less is that with a pressing pressure (linear pressure) of about 600 to 1000 kgf/cm, wrinkles can occur on the surface of the ceramic green sheet in some cases, thereby deteriorating the effect of decreasing surface roughness.
As described above, calendering under temperature and pressure (linear pressure) conditions in the ranges suitable for the ceramic green sheet can securely achieve the effect of smoothing the surface of the ceramic green sheet without causing separation of the ceramic green sheet from the carrier film, and breakage of the ceramic green sheet.
In the method of producing a ceramic green sheet, the smoothing is performed so that the surface roughness (Ra value) of the ceramic green sheet is about 100 nm or less.
In a multilayer ceramic electronic part in which each of the ceramic layers interposed between internal electrodes has a thickness (element thickness) of 3 xcexcm or less, the use of the ceramic green sheet having surface roughness (Ra value) of over about 100 nm for manufacturing the part has the tendency that the lifetime abruptly decreases. By applying the present invention, the surface (Ra value) roughness of a thin ceramic green sheet can be decreased to about 100 nm or less, and the durability of the multilayer ceramic electronic part manufactured by using the ceramic green sheet can be improved.
In the present invention, the surface roughness (Ra value) is determined based on measurements (nm) of a region of 5 xcexcm square obtained by using an atomic force microscope.
In the method of producing a ceramic green sheet of the present invention, the ceramic green sheet is separably held by a carrier film because ceramic green sheets are separated from the carrier films, and then laminated in use for manufacturing a multilayer ceramic electronic part.
The present invention is particularly useful for producing a thin ceramic green sheet for a multilayer ceramic electronic part required to have excellent surface smoothness. In the forming step of forming ceramic slurry in a sheet, and in the smoothing step of smoothing the surface, the ceramic green sheet is held by the carrier film, while in the lamination step, the ceramic green sheet can be separated from the carrier film. For example, a multilayer ceramic electronic part such as a thin layer multilayer-type monolithic ceramic capacitor or the like can be efficiently manufactured, in which thin ceramic layers are interposed between internal electrodes.
In manufacturing a multilayer ceramic electronic part by using ceramic green sheets produced by the method of producing a ceramic green sheet of the present invention, the surface roughness (Ra) of the interfaces between ceramic layers and internal electrodes can be decreased.
Smoothing the ceramic green sheet can increase the density of the ceramic green sheet, thereby decreasing the rate of occurrence of pores in a dielectric element of a capacitor.
Furthermore, since the density of the sheet is increased to suppress a sheet attack phenomenon in which the solvent component of an electrode paste permeates into the sheet to dissolve the sheet binder.
As a result, by using the ceramic green sheet produced by the producing method of the present invention, a multilayer ceramic electronic part having long life and excellent reliability can be manufactured.
The present invention is useful for producing ceramic green sheets for a multilayer ceramic electronic part in which the thickness (element thickness) of each of the ceramic layers is 3 xcexcm or less. By using ceramic green sheets produced by the producing method of the present invention for manufacturing a monolithic ceramic capacitor, for example, a large-capacity small monolithic ceramic capacitor having excellent electric properties and comprising a multilayer thin film can be efficiently manufactured.
In a still further aspect of the present invention, a method of manufacturing a multilayer ceramic electronic part comprises the forming step of forming ceramic slurry in a sheet, the smoothing step of pressing the formed ceramic green sheet to smooth the surface thereof, the sheet forming step of coating electrode paste for forming an internal electrode on the smoothed ceramic green sheet in a predetermined pattern to form a sheet provided with an electrode, and the lamination step of laminating the sheets provided with electrodes to form a lamination, and the burning step of burning the lamination.
The ceramic green sheet obtained by forming the ceramic slurry in a sheet is smoothed, and then the electrode paste for forming an internal electrode is coated in the predetermined pattern to form the sheet provided with an electrode. A plurality of the sheets provided with electrodes are laminated to form the lamination, followed by burning under predetermined conditions so that the multilayer ceramic electronic part can be efficiently manufactured while preventing deterioration in its life due to unevenness in the interfaces between internal electrodes and ceramic layers, and the occurrence of a structural defect (delamination, bending of an electrode portion or the like) in a multilayer thin film.
The process for smoothing the ceramic green sheet can improve the surface smoothness of the ceramic green sheet independently of the particle diameter and dispersibility of ceramic particles, thereby decreasing the surface roughness (Ra) in the interfaces between the ceramic layers and the internal electrodes of the manufactured multilayer ceramic electronic part.
Furthermore, since the density of the sheet is increased by smoothing, it is possible to prevent the sheet attack phenomenon in which the solvent component of the electrode paste permeates into the sheet to dissolve the sheet binder.
In the present invention, the ceramic green sheet is subjected to the smoothing process, and thus unlike a method for realizing the surface smoothness of the ceramic green sheet by using the high degree of dispersibility of ceramic particles, excessive shear force need not be applied to the ceramic particles during dispersion of ceramic slurry, thereby suppressing and preventing grinding of the ceramic particles. Therefore, it is possible to efficiently prevent the characteristics of the multilayer ceramic electronic part from deviating from the target ranges, or the characteristic values from becoming lower than the target characteristic values due to variations in agglomeration with ceramic particle lots. Specifically, for example, in a monolithic ceramic capacitor, it is possible to efficiently prevent the occurrence of a problem in which the design temperature characteristics are deviated from the target characteristics or only a capacity value lower than the design capacity can be obtained.
In the present invention, xe2x80x9cthe step of laminating the sheets provided with electrodes to form the laminationxe2x80x9d is a wide concept including not only a case in which only sheets provided with electrodes are laminated, but also a case in which sheets provided with electrodes are laminated and ceramic green sheets (outer layer sheets) provided with no electrode are laminated on upper and lower sides of the lamination of the sheets provided with electrodes to form a lamination.
In the present invention, xe2x80x9cthe ceramic slurryxe2x80x9d is a wide concept including not only slurry containing ceramic powder dispersed in a dispersion medium, but also slurry further containing additives such as a binder, a plasticizer, etc.
In the present invention, the ceramic green sheet is preferably obtained by forming the ceramic slurry in a sheet on a carrier film so that the ceramic green sheet held by the film is conveyed and subjected to the smoothing process. By smoothing the ceramic green sheet held by the carrier film, breakage of the ceramic green sheet in the smoothing process can be prevented even in the case of a thin ceramic green sheet, thereby efficiently producing a thin ceramic green sheet having excellent surface smoothness, a high density and high reliability.
In the method of manufacturing a multilayer electronic part of the present invention, the electrode paste for forming an internal electrode contains a base metal powder as a conductive component so that the internal electrode formed after burning the lamination comprises a base metal.
By using the ceramic green sheet subjected to the smoothing process, as described, even in forming internal electrodes (base metal internal electrodes) by using the electrode paste containing the base metal powder as the conductive component, it is possible to manufacture a multilayer electronic part which produces less deterioration of its life due to unevenness in the interfaces between the internal electrodes and the ceramic layers, and less structural defect (delamination, bending of an electrode portion or the like) in a multilayer thin film, thereby decreasing the electrode material cost without deteriorating reliability.
The present invention can be applied not only to a case in which the constituent material of the internal electrodes is a base metal, but also to a case in which a multilayer ceramic electronic part comprising internal electrodes made of a base metal is manufactured.
In the method of manufacturing a multilayer ceramic electronic part, the smoothing process is performed by using any one of the calender roll method, the plate pressing method and the hydrostatic pressing method.
As the smoothing method, any one of the calender roll method, the plate pressing method and the hydrostatic pressing method is used for smoothing the surface of the ceramic green sheet by pressuring it, thereby securely smoothing the surface of the green sheet and improving the smoothness of the interfaces between internal electrodes and ceramic layers. As a result, the pressure resistance, durability (life) and reliability of properties of the multilayer ceramic electronic part can be improved. The smoothing process by the above method can increase the sheet density to improve avoidance of defects such as pores of the ceramic layers. The calender roll method, the plate pressing method or the hydrostatic method preferably uses an equipment constructed so that the ceramic green sheet can be smoothed by pressurization under heating to a predetermined temperature.
The calender roll method uses a calender roll comprising, for example, a preheating roll (which may not be provided in some cases) for preheating the ceramic green sheet, and at least a pair of nip rolls (preferably comprising heating means) so that the ceramic green sheet is held between the pair of nip rolls and pressed from both sides to smooth the surface of the ceramic green sheet under pressure. The construction of the calender roll is not limited, and various types of calender rolls such as a single nip roll-type calender roll, a multi-stage nip roll-type calender roll comprising plural pairs of nip rolls, and the like, can also be used.
As the conditions for smoothing by the calender roll method, it is important to appropriately control the surface temperature of the nip rolls for nipping the ceramic green sheet and the linear pressure of the nip rolls. The surface temperature and the linear pressure are preferably controlled to the range of about 0 to 150xc2x0 C., and the range of about 50 to 1,000 kgf/cm, more preferably to the range of about 20 to 100xc2x0 C., and the range of about 100 to 600 kgf/cm, respectively.
The plate pressing method uses a plate press comprising, for example, a pair of parallel plates each comprising a mirror-polished hard chromium plated layer provided on the surface thereof, and a pressure control for controlling the pressing pressure of the pressing plates so that ceramic green sheet is held between the pair of pressing plates and pressurized from both sides to smooth the surface of the ceramic green sheet. The construction of the plate press is not limited. The pair of parallel plates is preferably constructed so that the surface of the sheet can be heated.
As the conditions for smoothing by the plate pressing method, it is important to appropriately control the surface temperature of the parallel plates, and the pressing pressure. The surface temperature and the pressing pressure are preferably controlled to the range of about 0 to 150xc2x0 C. and the range of about 500 to 10,000 kgf/cm2, more preferably the range of about 20 to 100xc2x0 C. and the range of about 1,000 to 6,000 kgf/cm2, respectively.
In smoothing by the plate pressing method, ceramic green sheets may be pressed one by one, or a plurality of sheets may be stacked and pressed. However, in order to prevent the ceramic green sheet from adhering to the pressing plates, the processed surface of a film subjected to separation processing is preferably attached to the surface of a ceramic green sheet, followed by pressing. In pressing a plurality of ceramic green sheets, in order to prevent each ceramic green sheet from adhering to the carrier film superposed thereon, the processed surface of a film subjected to separation processing is preferably attached to the surface of each ceramic green sheet, or a carrier film on the back of which is subjected to separation processing, is preferably used.
The hydrostatic pressing method uses a hydrostatic press comprising, for example, a pressure container filled with a liquid such as oil or water, a pressure cylinder for pressurizing the surface of the liquid, and a control for controlling the pressure applied to the liquid surface for smoothing the ceramic green sheet. The hydrostatic press is preferably provided with a liquid temperature control for controlling the temperature of the liquid such as oil or water to a predetermined temperature.
As the conditions for smoothing by the hydrostatic pressing method, it is important to appropriately control the liquid temperature (the surface temperature of the ceramic green sheet) and the pressing pressure. The liquid temperature and the pressing pressure are preferably controlled to the range of about 0 to 150xc2x0 C. and the range of about 500 to 10,000 kgf/cm2, more preferably the range of about 20 to 100xc2x0 C. and the range of about 1,000 to 6,000 kgf/cm2, respectively.
In smoothing the ceramic green sheet by the hydrostatic pressing method, for example, the ceramic green sheet is wound on the surface of a mirror-polished metal roll, packed with a flexible sheet under vacuum, and then immersed in the liquid such as oil or water of the hydrostatic press, followed by hydrostatic pressing. As a result, the ceramic green sheet is pressed on the surface of the metal roll and the film on the back of the ceramic green sheet under uniform pressure to smooth the surface of the ceramic green sheet.
In the method of manufacturing a multilayer ceramic electronic part of the present invention, the step of forming the sheet provided with an electrode comprises the step of coating electrode paste on the ceramic green sheet subjected to the smoothing process, and then drying the coating, and the step of coating ceramic paste containing a ceramic powder, a binder and a solvent on a region (sheet portion) of the electrode paste-coated surface, in which the electrode paste is not coated, and then drying the coating.
In a case in which only the electrode paste is coated on the ceramic green sheet (i.e., the ceramic paste is not coated on the region (sheet portion) of the ceramic green sheet, in which the electrode paste is not coated), the following problems occur in some cases:
(1) A step of about 1 xcexcm per layer occurs in the region between the electrode paste-coated portion and the uncoated portion, and thus lamination of hundreds of ceramic green sheets causes a step of about 100 to 500 xcexcm in the entire lamination. This step brings about bending of an extraction electrode portion for connecting an internal electrode and external electrode in the step of pressing the lamination, thereby causing a short-circuit defect; and
(2) The step causes distortion of the structure of a capacitor, thereby easily causing delamination during burning.
However, in the present method of manufacturing a multilayer ceramic electronic part, the ceramic paste is coated on the region (sheet portion) of the ceramic green sheet in which the electrode paste is not coated, and then dried to form the ceramic green sheet, without forming the above step. Therefore, it is possible to decrease the structural defects of the multilayer ceramic electronic part, such as short-circuit defects, delamination, etc. It is also possible to prevent breakage of the internal electrodes due to the step, thereby improving reliability.
The method of manufacturing a multilayer ceramic electronic part further comprises smoothing (secondary smoothing) the ceramic green sheet subjected to the smoothing process (primary smoothing) after the electrode paste and the ceramic paste are coated thereon and dried.
After the electrode paste and the ceramic paste are coated on the ceramic green sheet subjected to the smoothing process (primary smoothing) and dried, the ceramic green sheet is further smoothed (secondary smoothing), thereby removing print deviation, coating waviness during printing and a saddle phenomenon. Therefore, surfaces of the electrode paste coating and the ceramic paste coating can be further smoothed, and the densities thereof can be increased. As a result, the smoothness of the interfaces between the internal electrodes and the ceramic layers in the multilayer ceramic electronic part can be improved to improve pressure resistance. Also, the occurrence of structural defects (delamination, bending of an electrode portion, etc.), which are easily produced in multilayer films, can be suppressed and prevented to effectively manufacture the multilayer ceramic electronic part having high reliability.
The method further comprising coating the ceramic paste and performing the secondary smoothing process is particularly useful for manufacturing a multilayer ceramic electronic part in which the thickness (element thickness) of each of the ceramic layers is 3 xcexcm or less. For example, in application to the manufacture of a large-capacity small monolithic ceramic capacitor comprising a multilayer film, a monolithic ceramic capacitor having excellent electric properties and light reliability can be efficiently manufactured.
In the method of manufacturing a multilayer ceramic electronic part of the present invention, the secondary smoothing process is performed by any one of the calender roll method, the plate pressing method and the hydrostatic pressing method.
By using any one of the calender roll method, the plate pressing method and the hydrostatic pressing method for the secondary smoothing process, the surfaces of the electrode paste coating and the ceramic paste coating applied to the surface of the ceramic green sheet are securely smoothed to improve the smoothness of the whole sheet provided with an electrode, thereby making the present invention more effective.